Good permanent magnet materials must have a large remanent magnetization, large coercive field, and high Curie temperatures. This indicates the best candidates to be rich in 3d transition metals, allowing large magnetic moments and strong magnetic interactions, and to have non-cubic crystal structures, allowing strongly anisotropic magnetic properties. The strong spin-orbit coupling associated with the 4f electrons of rare-earth elements can lead to enhanced intrinsic (magnetocrystalline) anisotropy, and the best permanent magnet materials contain rare-earths in combination with 3d transition metals. However, there is economic and scientific interest in attaining good permanent magnet properties without rare-earth elements.
AlNiCo, a well-known, conventional, non-rare-earth magnet, has relatively weak magneto-crystalline anisotropy, and attains good performance through anisotropic microstructures (shape anisotropy) developed during spinodal decomposition in an applied magnetic field.
Another high-performing, alternative material is PtCo. Strong spin-orbit coupling on Pt is important in providing magnetic anisotropy. Spin-orbit coupling is strong in all heavy elements, and combining 3d transition metals with heavier 4d/5d metals is a contemplated route to new anisotropic ferromagnets. However, the use of precious and/or semiprecious metals reduces the attractiveness of these materials.
There is a need for high performance, non-rare-earth magnetic materials that contain little or no precious and/or semiprecious metals.
In the ferromagnetic state, the coercivity or coercive field is defined as the magnetic field at which the magnetic moment of a magnetized sample is reduced to zero. If mechanisms are available with little energetic barrier to either rotate the moment within a magnetic domain, or nucleate a reversed domain and move the resulting domain wall, coercivity will be low. Such a material is referred to as a soft ferromagnet. If this is not the case, and there is high resistance to demagnetizing fields (i.e. rotating magnetic moments within a domain is energetically costly, and/or magnetic domain walls are prevented from moving freely), then the materials is referred to as a hard ferromagnet. In general, materials with coercivity ≧1000 Oe can be classified as hard ferromagnets, and are required for permanent magnets. Soft ferromagnets have lower coercivity, and good soft ferromagnets have coercivity <1 Oe, and are important for applications like transformer cores. Intermediate materials having a coercivity >1 Oe and <1000 Oe are useful in applications where a magnetic hysteresis losses are required in applications such as, for example, transformation of electromagnetic energy into thermal energy, also known as magneto-thermal conversion.
In all cases, microstructure plays a key role in magnetic performance by determining the mechanisms by which the reversal of magnetic moments occurs. As noted above, the microstructure of AlNiCo magnets results in strong anisotropy and ultimately high coercivity. In rare-earth magnets, the high intrinsic magnetocrystalline anisotropy leads to high coercivity only when proper microstructure is realized.
Melt-spinning is a commercially used process for obtaining the fine-grained microstructures required for hard ferromagnets. The rapid cooling that occurs during this process can produce kinetically stabilized microstructures far from equilibrium. Controlling the subsequent evolution by relaxation of these high energy states enables fine tuning of the microstructures.